1. Field of the Invention
The invention relates to an optical fiber device for use in communication and sensors. The device allows transmission of a light signal through an optical fiber in one direction, but prevents its propagation in the opposite direction.
2. Information Disclosure Statement
Optical fiber components and sensors have become increasingly more important for effectively transmitting and processing signals in optical communication systems and many different optical fiber devices. Optical fiber systems essentially consist of at least one light transmitting optical fiber, a signal processing or sensor component, and a coherent radiation source (a laser). One of the most important tasks of many optical fiber systems is to prevent the light signal from reflecting back to the laser source. The properties of a laser can be greatly influenced by this undesired signal. This problem is avoided using an isolator in the optical fiber circuit to transmit the light in one direction, but prevent its propagation in the opposite direction.
A frequently used optical fiber isolator consists of a bulk magneto-optical crystal affected by a magnetic field. The isolator is placed between two appropriately adjusted polarizers and optically connected between an interrupted optical fiber (see e.g., Laser Focus v.14, No .11, p. 52 (1978)). The use of an isolator in an integrated optical design which employs a waveguide is also known. The waveguide is produced from a magneto-optical film instead of a bulk crystal which reduces the magnitude of the control fields required (see e.g., U.S. Pat. No. 4,859,013).
Although these isolators are effective, they face several shortcomings. Traditionally, when using isolators of this type, the optical fiber is interrupted and the isolator spliced in line using butt-joint connections. The need to interrupt the optical fiber circuit and the optical losses associated with such an interruption are the main disadvantage of many known optical isolators. Another disadvantage is that these isolators are designed for a specific wavelength, and it is difficult to change their properties quickly when switching to another light wavelength.
All-fiber in-line isolators which employ Faraday rotation in a coil fabricated from birefringent monomode fiber placed in a magnetic field (see J. Lightwave Techn. v.27 p.56 (1984)) are also burdened with the need for external polarizers operating at some specific wavelength. A new kind of all-fiber in-line component based on an evanescent coupling between a single-mode fiber and a multimode planer waveguide overlay has recently been investigated and used to design an all-fiber in line wavelength selective element and intensity modulator (see Electronics Lett. V. 28, p. 1364 (1992)). This device, however, cannot perform isolation functions.